Fig. 1. DPAL system in LLNL^[44]. (a) 34 kW DPAL apparatus; (b) new generation of DPAL system

Fig. 2. DPAL system in AFRL^[46]. (a) Layout of the laser system; (b) design of the atom cell

Fig. 3. DILAS semiconductor pump source^[47]. (a) Vertical bar array packaging; (b) fiber-coupled laser diode

Fig. 4. DPAL apparatus in Russia and Japan. (a) kW-level Cs laser in Russia^[16]; (b) DPAL experimental platform in Tokai University, Japan^[48]

Fig. 5. DPAL design with the pump laser, output laser, and gas flow vertical to each other

Fig. 6. DPAL sapphire window based on micro/nano surface^[65]. (a) Physical image; (b) enlarged picture of micro/nano surface

Fig. 7. Energy level and transition process of DPRGL

Fig. 8. Several representative discharge setups of DPRGL. (a) Pulse DC discharge^[70]; (b) microwave micro discharge^[71]; (c) radio frequency dielectric barrier discharge ^[72]

Fig. 9. DPRGL research platform in National University of Defense Technology^[74]. (a) DPRGL platform based on the pulsed DC discharge; (b) atmospheric large volume plasma jet

Fig. 10. DPRGL power scaling scheme based on a multi-jet gain array^[76]

Fig. 11. Principle of the EUV light generation by optical pumping of rare gas atoms^[77]